This invention is in the field of surgically implantable blood pumps that can be used to augment or replace the pumping action of the heart.
Several types of surgically implantable pumps have been developed in an effort to provide mechanical means for augmenting or replacing the pumping action of damaged or diseased hearts. Some of these pumps are designed to support single ventricular function. Such pumps usually support the left ventricle, which supplies the entire body except the lungs, since it becomes diseased far more commonly than the right ventricle, which supplies only the lungs. Other devices have been tested and used for providing biventricular function. As used herein, "testing" refers to tests on animals (such as dogs or cows), while "use" refers to implantation in humans.
Depending on the needs of a particular patient and the design of a pump, pumping units such as so-called "LVAD's" (left ventricular assist devices) can be implanted to assist a functioning heart that does not have fully adequate capacity. Other types of pumps, such as the so-called "Jarvik heart," can be used to completely replace a heart which has been surgically removed.
Temporary as well as permanent implantable pumps have been developed. "Permanent" in this sense refers to the remaining life of the patient; after a patient's death, any artificial pumping device is usually removed for analysis. "Temporary" implantation usually involves (1) an attempt to reduce the stress on a heart while it recovers from surgery or some other short term problem, or (2) use of a pump as a "bridge" to forestall the death of a patient until a suitable donor can be found for cardiac transplantation.
Throughout the remainder of this application, the term "blood pump" or "pump" refers to, and is limited to, surgically implantable pumps. Such pumps do not include pumps which remain outside the body of a patient. For example, the pump in a conventional cardiopulmonary bypass machine, used during most types of heart surgery, would not be covered by the specification or claims herein. For convenience, all references to "aorta" refer to the ascending aorta. The descending portion of the aorta, which passes through the trunk and abdomen, is not relevant to this invention.
The most widely tested and commonly used implantable blood pumps employ variable forms of flexible sacks (also spelled sacs) or diaphragms which are squeezed and released in a cyclical manner to cause pulsatile ejection of blood. Such pumps are discussed in books or articles such as Hogness and Antwerp 1991, DeVries et al 1984, and Farrar et al 1988, and in U.S. Pat. Nos. 4,994,078 (Jarvik 1991), 4,704,120 (Slonina 1987), 4,936,758 (Coble 1990), and 4,969,864 (Schwarzmann et al 1990 ). Sack or diaphragm pumps are mechanically and functionally quite different from the present invention.
An entirely different class of implantable blood pumps uses rotary pumping mechanisms. Most rotary pumps can be classified into two categories: centrifugal pumps, and axial pumps. Centrifugal pumps, which include pumps marketed by Sarns (a subsidiary of the 3M Company) and Biomedicus (a subsidiary of Medtronic, Eden Prairie, Minn.), direct blood into a chamber, against a spinning interior wall (which is a smooth disk in the Medtronic pump). A flow channel is provided so that the centrifugal force exerted on the blood generates flow.
By contrast, axial pumps provide blood flow along a cylindrical axis, which is in a straight (or nearly straight) line with the direction of the inflow and outflow. Depending on the pumping mechanism used inside an axial pump, this can in some cases reduce the shearing effects of the rapid acceleration and deceleration forces generated in centrifugal pumps. However, the mechanisms used by axial pumps can inflict other types of stress and damage on blood cells.
Some types of axial rotary pumps use impeller blades mounted on a center axle, which is mounted inside a tubular conduit. As the blade assembly spins, it functions like a fan, or an outboard motor propeller. As used herein, "impeller" refers to angled vanes (also called blades) which are constrained inside a flow conduit; an impeller imparts force to a fluid that flows through the conduit which encloses the impeller. By contrast, "propeller" usually refers to non-enclosed devices, which typically are used to propel vehicles such as boats or airplanes.
Another type of axial blood pump, called the "Haemopump" (sold by Nimbus) uses a screw-type impeller with a classic screw (also called an Archimedes screw; also called a helifoil, due to its helical shape and thin cross-section). Instead of using several relatively small vanes, the Haemopump screw-type impeller contains a single elongated helix, comparable to an auger used for drilling or digging holes. In screw-type axial pumps, the screw spins at very high speed (up to about 10,000 rpm). The entire Haemopump unit is usually less than a centimeter in diameter. The pump can be passed through a peripheral artery into the aorta, through the aortic valve, and into the left ventricle. It is powered by an external motor and drive unit.
Centrifugal or axial pumps are commonly used in three situations: (1) for brief support during cardiopulmonary operations, (2) for short-term support while awaiting recovery of the heart from surgery, or (3) as a bridge to keep a patient alive while awaiting heart transplantation. However, rotary pumps generally are not well tolerated for any prolonged period. Patients who must rely on these units for a substantial length of time often suffer from strokes, renal (kidney) failure, and other organ dysfunction. In addition, rotary devices, which usually must operate at relatively high speeds, may impose high levels of turbulent and laminar shear forces on blood cells. These forces can damage or lyse (break apart) red blood cells. A low blood count (anemia) may result, and the disgorged contents of lysed blood cells (which include large quantities of hemoglobin) can cause renal failure.
One of the most important problems in axial rotary pumps in the prior art involves the gaps that exist between the outer edges of the blades, and the walls of the flow conduit (see FIG. 3-B). These gaps are the site of severe turbulence and shear stresses, due to two factors. Since implantable axial pumps operate at very high speed, the outer edges of the blades move extremely fast and generate high levels of shear and turbulence. In addition, the gap between the blades and the wall is usually kept as small as possible, to increase pumping efficiency and to reduce the number of cells that become entrained in the gap area. This can lead to high-speed compression of blood cells as they are caught in a narrow gap between the stationary interior wall of the conduit, and the rapidly moving tips or edges of the blades.
In order to reduce turbulence and shear around the outer edges of the blades, and to strengthen and reinforce the blades, large pumping units used for pumping oil or water often use a circular rim attached to the outer edges of the blades. However, to the best of the Applicant's knowledge, such rims are not used in axial blood pumps, since such rims would increase laminar shear between the rotating rim and stationary cylinder walls, and would also contribute to stasis between the two surfaces.
Despite their disadvantages, axial pumping devices remain of great interest, since they are smaller and less complex than sack or diaphragm pumps. Axial pumps do not need inlet or outlet valves, they have fewer and smaller blood-contacting surfaces, and they can be surgically implanted much more easily than sack or diaphragm pumps.